Highlight aperture correction system



M. v.y SULLIVAN HIGHLIGHT APERTURE CORRECTION SYSTEM Nov. 28, 1961 5 sheets-sheet@ Filed Sept. 26, 1956 INVENTOR M ICHAEL V. SU LLVAN Y @Lw/Mfm HIS ATTORNEYS N0V 28, 1961 M. v. SULLIVAN 3,011,018

HIGHLIGHT APERTURE CORRECTION SYSTEM Filed sept. 26', 195e s sheets-sheet 2 F`IG.3.

' INVENTOR MICHAEL V. SULLIVAN H IS ATTORNEYS Nov. 28, 1961 M. v. SULLIVAN HIGHLIGHT APERTURE CORRECTION SYSTEM Filed Sept. 26, 1956 3 Sheets-Sheetl 3 INVENTOR MICHAEL V.SUI LIVAN 21mm I yHlsA Tas United States invention relates to television, and more particularly to a new andv improved aperture equalization method and ineens for correcting aperture distortion in the highlight .regions of .the television pieturewithout increasing the high frequeneynoise which .is inherent in the shadow areas of the picture information signal.

The eieet .known as .aperture distortion hes'long been enscunteredin the transmission .and .reproduction of en .image hy television. .end by similar systems utilizing inzage scanning ineens.. distortion arises frein the use of a .scanning .spot of finite size which produces e response proportional to :the vaverage light intensity with.- in the .image ereg 4ctn/ered by .the .spot et .any instant. As e consequence, the .sharpness of image borderrines ci high contrast ,is reduced, resulting in e degradation of resolution end picture quality in the reproduced pie- .'ture.

To overcome the .aperture distortion inherentin television systems, an aperture equalizer comprising a l{trans- :versal filter-:having .e .substnntiellv .linear phase-frequency characteristic .is .customer-ily used.. .Such aperture "equalizers present niinimurn irnpedgnee to the frequencies rep.- resented by the rate of travel of the scanning spot over theelernentnlarens .of the mosaic oi the pickup tube, and higher .impedance to other frequencies. Differences .image intensity between the adjacent elemental 4areas of the mosaic are thus enhanced, resulting in en irn- .provement in borderline contrast resolution in the re.- tproducedpicture. Aperture equalizers .of this type, how.- ever; alsov present .o low impedance to high frequency noisesignuls which are concentrated in the signal amplitude levels corresponding to the low .brightness or blackto-gray picture areas, resulting in degradation .Gf .Cl-.llality in the shadow-areas of .the picture.

@It is an object of this invention., accordingly, to provide `a new and improved aperture equalization method and means for television systems in which aperttue distort-ion is corrected only -inV the highlight areas `of the picture, where the human eye perceives greater detail than inthe shadow areas.

;-It is another object of this invention to provide a .new .and improved aperture equalization method Aand means for television systems whereby the `gradient be- .tween adjacent elements of a high frequency information signal of varying amplitude'may be restored after degradation, without increasing the noise level.

These and other objects of the invention are accomp lished by dividing the picture information signal into two parts with respect to amplitude, applying the signal portion representing the gray-'to-white region to an aper- `ture equalizer, and recombining vthe two portions of pic- ,turc vinformation signal so that the resulting signal is corrected foraperture distortion only in the gray-to-white .preps Of the picture, while `the noise signals associated r.with the lo'w brightness or black-to-.gray portion of the information signal are not appreciably increased.

Further objects and advantages of the invention will .be apparent to those skilled kin the .art Ifrom a reading of-the following .detaileddescription of a typical Aembodifment, taken withrefercnce to the accompanying drawings., in which:

FIG. 1 is a block diagram of a typical highlight aperture tequaligation :system constructed according to the invention;

.arent f sonnig patented Nov. 2,18, 1.961

2 FIG. 2 illustrates theswaveform of the television picture signal at various points vin the aperture equalization system shown in FIG.' 1; f

FIG. 3 is a circuit diagram of the input and white clipper portions of the system shown in FIG. l;l L

FIG. 4 is a circuit diagram of. the difference empliier portion of the systemd shown in FIG. l; and

FIG. 5 is a circuit diagram of thesumming amplifier and output portions .'of the. system shown in FIG. `1.

Referring iirsi .to the bleek diegrern of PIG- 1 and the graphic representations of the'Vv signal waveform of FIG. 2 a television picture information signal as illustrated in FIG. 2q is applied through onev conductor, preferably a coaxial cable, to a so-called white'clipper A, whieh posses only those portions of the input signal below a predetermined amplitude level .corresponding to a predetermined bleek-toeter region of the picture, as shown FlG- 2b,- The "output" .signal from the wlnte clipper A 12b.) is applicd to a difference amplifier.

B which also receives the'full range picture information c signal (FIG, A delay lineC may be interposed between the signal input and the difference amplifier B to compensate for any delay introduced by the y white elippe.r A.-

The output .signal from the diferenee amplifier B, whieh .hes the waveform shown .in FIG 2e, is-the differentre. between the waveforms shown in ,FlCiS- .2o end 2b. and represents only the gray-.to-white picture .information. signal is pessedthrough an aperture equalizer l), which may be a ,conventionalelcctrical wave filter of the type described in .United States Patent No. 2,273,163, for example. The aperture equalizer D -functions to increase the slope of thefarnplitude changes in the signal, thus compensating ,for contrast .degradation resulting from the lfinite size of the scanning spot and reproducing the sharpness of the `image borderlines of high contrast in the highlight areas as it occurred in the original picture. FIG. 2d illustrates the Waveform of the signal after aperture distortion v.correction `by -the aperture equalizepIl c The aperture'equalized gray-to-white signal and the black-to-gray signal from the white clipper A are recombined by a summing amplifier Ev into a full-range television picture signal having the waveform shown in FIG. 2e.. A conventional delay line F vor equivalent `devicernay be interposed between'the white clipper A and the summing ampliiier E, if necessary, to compensate vfor lthe retardation of the gray-towhite signal by the aperture equalizer D Referringnow to FIG. 3, which illustrates a practical c form of aperture equalization system according to the invention, ythe composite video signal input is supplied to an input terminal 10 and is amplified in a conventional single stage amplifier including the pentode 16. The amplifier input circuit comprises a resistance 11, one end vof which is connected to the ground conductor 13, a gain control potentiometer 12 having an adjustable tap 14 connected .to the control grid electrode 15 of the tube 16 through a series condenser 17 and shunt resistor 18 oombination.

The tube 1,6 may be one section of a dual tube such as the tube type 6AN8, for example. It has a suppressor grid electrode 19 and a cathode electrode 20 connected together and to ground through a resistor 21, and a screen gridelectrode 22 connected to a B+ conductor 23 through a resistor 24 and to ground through a condenser 25. A-plate electrode 26 .is connected to the B+ conductor 23 through a series pla-te load impedance comprising a resistor v27, a choke 28 and another resistor 29, the junction of thechoke 28 and the resistor 29 being c connected to ground through a condenser 3.0. If desired,

the gain 'of this stage maybe changed by vary-ing the setting of the adjustable tap 14 of the potentiometer 12.

The output of the tube 16 is supplied from the plate electrode 26 to the difference amplifier B of FIG. l, which is illustrated in detail in FlG.V 4,through a terminalV 31, and to the control grid 32 of a'triode 33; which constitutes tube, for example. The plate 91 of the triode 78 is conpart of the white clipper A (FIG. 1)l through a con- ,n

denser 34.

The ycontrol grid 32 of the vtriode 33 is clamped at a horizontal rate by clamping pulses synchronized with the horizontal drive pulses of the videosignal. The clamping pulses may be generated by vaV typical; clamping circuit such asthe one illustrated generally at'36 comprising two Vtrviodes '37 and 38,'a transformer '39 and a vvoltage divider consisting of two V'resistors 40v and 41k connectedx between theBV-- conductor'and ground.- The'two' triodes 37 and 38 may eachbe'the 4triode portions of dual-type triodepentode tubes such as the 6AN8, for, example. lN eg'native horizontal scan drive pulses are applied at an input terminal 42 tio generate the clamping pulses.

The construction and manner of'operation of clamping circuits of this type is well known and need not be described in detail herein. Suice it to say that the clamping pulses occurduring theblanking time ofthe composite video signal immediately after'each synchronizing pulse. One set of pulses'in positive polarity is applied to the plate 44 of one diode and another set is applied in negative polarity to the cathode 45 of the other diode.' The diodes are thereby caused to conduct and they'connect the grid 32 of the tube33 toaiixed bias voltage determined by the voltage divider comprising the resistors 40 and 41.

The cathode electrode 46 of. the'triode 33 is connected lto ground through acathoderesistor y47 and 'the plate 35 is directly connected totheV positivef terminal of the plate supply. Itlwillfbe understood that the output at the cathode y46 is the composite video signal tofwhich a D.C. component has been restored. This output lis 'fed to the electrically connected plate lelectrodes 48 of a -double diode 49, which serves asa signal clipper, the

electrically connected cathode electrodes 50 of which are connected toV groundthrough the resisto'rsSl and 55, the latter being byY-pafs'sed'to" ground'by a; condenser 54. The clipped video signal appearing at the cathode electrodes 50 of the diode 49 is supplied to the difference amplifier B of FIG. l, which is illustrated in' detail in FIG. 4, through .a terminal 5'6. Y

The clipping level is determined bythe bias voltage developed acnoss, the resistor 55 which is connected to the cathode 52 of a triode 53 having its plate 62directly connected to the positive terminal of the plate supply voltage. The bias voltage is adaptedto be varied by adjustment of a potentiometer 57 comprising part of a voltage divider including the resistors 5S and 59, the potentiometer S7 having 4an adjustable tap connected in series with a resistor '61 tothe control grid 60 ot the tube 53. The control grid 60 is by-passed to ground by a condenser 63. l

tlf desired, the two triodes 33 and 53 may be sections of a dual-type tube such as the 6BZ7, for example, and the double diodes 43 and 49 may he tubes of the 6AL5 type.

l In order to invert the clipped signal and provide a one volt signal tothe input of a 75 ohm delay line 74,

. the clipped signal at the cathodes 50 of the diodes 49 is fed through a condenser 67 to the control grid 65 of a pentode 64, a resistor 66 being connected between the grid 65 and ground. rIhe pentode 64, which may be of v the 6CL6 type, for example, has its suppressor grid electrode 68 and its cathode electrode 69 connected together and to ground through a resistor 70,. the plate electrode 71 beingconneeted to the screen grid electrode 72 .and to-the B+ .conductor 23'through a resistor 73. y

v The portion of the composite video signal representing only gray-to-white is obtained by supplying the composite signal to a difference ampliiier of the type shown in FlG.

' output at the terminal 74 in FIG. 3.

nected to the B+ lead 23 and the cathode 89 is connected in series with a potentiometer 87 to the plate 88 of the triode 77. The grid 90 ofthe triode 78 receives the clipped signal representing black-to-gray at the terminal56. f l .l

The full range video signal is applied to the control grid S1 of the triode 77 through an input circuit including a series condenser 82, a shunt resistor S3 connected to ground, and a series resistor 84, the cathode electrode 85 lof the amplilier also being connected to ground through aresistor 86. f- 5 The diierence signal appearing at vthe plate electrode 88 of the triode 77 is applied to the control gnd elecjtrode 92 of a pentode 93 through a series condenser 94, the control grid electrode 92 being connected to ground through a resistor 9S. The pentode 93, which mayV be of the 6CL6 type, has a suppressor grid electrode 96 and a cathode electrode 97 connected together and to ground througha resistor 98, the plate electrode 99 being connected to the screen grid electrode 100 and to the B+ conductor 23 through a resistor 101. The inverted difference signal is applied through aseries condenser 102 and a'shunt resistor 103 to one terminal 104 of a conventional -aperture equalizer 10411, which may -be of the type described in United States Patent No. 2,27 3,163 ,i for example, the output terminal 104b of which is connected to the summing amplifier shown in det-ail in FIG. 5 by a conductor 104e. l i

'I'he summing amplifier comprises a'pair of triodes v105 and 106, which may be in a common envelope as in the type 6BZ7` tube, for example, having control grid electrodes 107and 108. Y

The grid electrode 108 is connected tothe junction of two resistors 121 and 122 forming a voltage divider connected to the B+ lead 23 and `grouncLand in series with a condenser 112 to the terminal 110 of a conventional delay line l10n, a resistor 120 being connected between the terminal 110 and ground. The terminal 1101 of the delay Vline 110a is connected to receive the clipped The delay line 1100 serves to introduce suliicient delay to compensate for the delay introduced by the aperture equalizei 104e in FIG; 4.

The grid electrode 107 is connected in an input circuit including the series condenser 111, the shunt resistors 113 and 11S and the terminal 109 which is connected to receive the output at thev terminal 10411 of 'the aperture equalizer 104:1 (FIG. 4) through the conductor 104e.

YA potentiometer 117 is connectedbetween the plate electrode 118 of the triode 105 and the cathode electrode 119 of the triode 106 to provide adjustment of the v gain of system in order to assure an accurate recombination of the two signals. Also, the plate electrode 123 of the triode 106 is connected directly to the B+ conductor 23.

The recombined video signal is amplified by a pentode 124 having a control` grid electrode 1,25 connected to ground through a resistor 126 and to the plate electrode 118 of the triode 105 through a condenser 127. The cathode electrode 123 andthe suppressor grid electrode 129 are connected together and to ground through a common resistor 130. The screen grid electrode 131 is connected to the B+ conductor 23 through a resistor 132 and to ground through a condenser 133. The plate electrode 134 is connected to the B+ conductor through a tude (e.g., one volt) into the impedance represented by the next input stage, which may be, say, 75 ohms, a. pentode 139 such as a 6CL6 tube, for example, may be utilized. It has a control grid electrode 140 connected to ground through a resistor 141 and to the plate electrode 134 of the pentode amplifier 124 through a condenser 142. The cathode electrode 143 and the suppressor grid electrode 144 are connected together and to ground through a resistor 145, the plate electrode 146 Ibeing connected to the B+ conductor 23. through a plate load resistor 148. The recombined aperture-equalized signal appearing at the plate electrode 146 may be ap-` plied toan output terminal 149 through a condenser 150, the terminal 149 being shunted to groundk through a resistor 151.

In considering the operation of the novel highlight aperture equalizing system described above, it will be observed that the action of the clamping circuit 36 and the dou-ble diode 43 maintains a constant 'blanking level so` that the picture signal will always be clipped at the desired point in the shadow-to-highlight ran-ge.l of the signal.

Adjustment of the clipping level to any desired point in the brightness range may be made by varying the setting of the potentiometer 57 (FIG. 3)., This; changes the bias of the control grid 60 and causes more or less current to pass through the resistor 55, thereby increasing or decreasing the bias of the cathode. electrode- 50 of the double diode 49.

It is assumed that the input video signal applied t'o the amplifier 16 is in the yform shown in FIG. 2a. This signal is inverted in the amplifier 16 so that at the control grid 32 of the triode 33 and at lthe output terminal 31 the higher signal amplitudes correspond to lower brightness levels. Since the tube 33 is connected in a cathode-follower arrangement, no signal inversion occurs, and the clipped signal appearing at the cathode electrode 50 and at the output terminal 56 remains inver-ted with respect to brightness.

It will be noted that the difference amplifier (FIG. 4) is arranged so that an increase in potential at the control grid 81 of the triode 77 decreases the potential at t-he control grid 92 of the tube 93, whereas an increase in the potential at the control grid 90 of the triode 78 has the reverse effect. Further, the potentiometer 87 is adjusted so that when identical signals are applied to the grids 81 and 90, respectively, of the two triodes 77 and 78, no signal will appear at the control grid 92. Under these conditions, when a full-range video signal is applied to the triode 77 and a clipped signal is fed to the triode 78, a difference signal, inverted with respect to amplitude, will appear at the control grid 92 of the tube 93.

As mentioned above, both the signals applied to the difference amplier are inverted with respect to amplitude so that the difference signal thus produced is erect in form, the higher signal amplitudes corresponding to greater image brightnesses. This signal is again inverted by the action of the amplifier including the tube 93,. so that an inverted signal appears at the output terminal 104.

Similarly, the gains of the summing amplifier triodes 105 and 106 are adjusted by means of the potentiometer I117 so that when identical signals are applied to the control grids 107 and 108 (FIG. 5) no signal is produced at the grid 125 of lthe tube '124. Hence, the clipped black-to-gray signal, after further inversion in the amplifier including the tube 64 (FIG. 3) is impressed on the control grid 108 of the tube 106 (FIG. 4) in erect form with respect to brightness, while the aperture equalized difference signal, consisting of gray-to-white information only, is applied to the control grid 107 in inverted form. As a result, an erect, full-range video signal is produced at the grid 12S of the tube 124. The action of the summing Iamplifier, therefore, is to recombine the dlipped black-to-gray signal and the aperture-equalized gray-towhite signal into a full range television picture signal.

The recombined signal is amplified by the tube 124 (FIG. 5), the resulting inversion being corrected by the tube 139 so that a full range video signal, erect with respect to brightness and aperture-equalized in the grayto-white region, is applied to the output terminafl 149.

The invention thus provides a novel and highly effective aperture equalization method and means which -corrects for aperture distortion in the important gray-towhite regions of the picture where the human eye is most sensitive, without materially increasing the noise level which predominates in the black-to-gray region.

The 'specific embodiment herein described is intended to be merely illustrative and not restrictive ofthe invention. Various modifications and changes in form and detail will be obvious to those skilled in the art within the scope of the Ifollowing claims.

'I claim:

1. In a method for reproducing visible information by means of a video signal varying in value as a function of a quality of lsuccessively scanned elemental areas of said visible information and subject to aperture distortion effects, the steps of deriving from said video signal components representing variations of said video signal in different ranges of amplitude values, respectively, emphasizing higher frequency components in one of said video signal components representing -a given range of amplitude values to correct for aperture distortion caused by scanning of said elemental areas, and recombining said video -signal components to produce a complete video signal similar to said first-named signal but corrected for aperture distortion in said given range of amplitude values.

2. In a method for reproducing visible information by means of a video signal varying in amplitude as a function of the brightness of successively scanned elemental areas of said visible information and subject to aperture distortion effects, the steps of deriving from said Video signal components representing different ranges of amplitude variation of said vide-o signal, respectively, emphasizing high frequency components i-n one of said signal components representing brighter elemental areas of said visible information to correct for aperture distortion caused by scanning of said elemental areas and combining said video signal components to form a complete video signal similar to said first-named video signal but corrected lfor aperture distortion in a range of amplitude variation representing said brighter elemental areas of said visible information.

3. In a system for reproducing visible information by means of a video signall varying in value as a function of a quality of successively scanned elemental areas of said visible information and subject to aperture distortion effects, the combination of means for deriving from said video signal a first video signal component representing variations of said video signal in a first range of amplitude values, means for deriving from said video signal a second video signal component representing variations of said video signal in a second range of amplitude values, means for correcting one of said signal components for the effects of aperture distortion caused by scanning said elementall areas, and means for combining the corrected signal component and the other of said signal components to form a complete video signal aperture corrected in a given region of amplitude values corresponding to a given range of variation of said quality.

4. In a system for reproducing visible information by means of a video signal varying in amplitude as a function of the brightness of successively scanned elemental areas of said visible information and subject to aperture distortion effects, the combination of means for deriving from -said video signal a first video signal component representing variations of said video signall in a first range of amplitudes, means `for deriving from said video signal a second video signal Icomponent representing variations of said' video signal in a second range of amplitudes, means for correcting one of said signal components for thefefects of aperture distortion caused by scanning said elemental areas, 'and means for combining the corrected signal component and the other of said signal components to form a complete video signalaperture corrected in agiven'region of amplitudes corresponding to a given brightness range of variation of said quality.

5. In va television system including imageY scanning means subject to aperture distortion, the combination of electrical means for dividing a television picture signal into atleast twoelectrical signal components representing selected brightness level. rangesof picture information, respectively, aperture equalizing means for correcting aperture distortion in at least one of said signal components, and electrical means for combining said corrected electrical signal component and the other of said signal components to form a complete television picture signal.

6. In a television system including image scanning means subject` to aperture distortion eiects, the combnation of signal( clipper meansvadapted to receive a television composite video signal and to clip amplitudes thereof above a predetermined value, means responsive to said video signal and to the voutput of said clipper means for producing'adiierence signal, means for emphasizing the higher frequency components of said difference signal to correct for ythe effects of aperture distor' tion, and means responsive to the outputs of* said emphasizing means and said signal; clipper means for producing 4a signal representative of the sum of said clipped signaland` said difference signal. v

7. In a television system including image scanning means subject to aperture distortion etects, the combinationof keyed clamp ampliiier means having input means adaptedrto receive a television composite video signal, signal clipper means for clipping amplitudes of the out- Y put of said amplier means above a predeterminedvalue,

References-Cited inthe file of this patent UNITED STATES rATENTs 2,691,696 Yule ocr. 12, 1954v 2,697,758 v Lime Deezl, 1954 2,740,912 Grahamv f Apr., 3, 1956 2,791,751 

